Maybe Piers is actually a wizard in disguise, sent to keep an eye on Harry in the Muggle world. But he ends up befriending Dudley for real. This would turn their whole dynamic on its head and create a lot of drama as they figure out what to do with this new information.
What if Dudley and Piers were actually kidnapped by a dark wizard who thought they could be used to get to Harry? Harry then has to rescue them, but in the process, Dudley and Piers learn some dark secrets about the wizarding world that they weren't supposed to know. This would change their perspective on Harry and his world forever.
A plot twist could be that Dudley has some latent magical ability that is awakened when he is in a life - or - death situation with Piers. This would completely change their relationship with Harry as now Dudley is also part of the magical world in a way.
One idea could be a story where Dudley and Piers start to see Harry in a new light after he saves them from a magical threat in the Muggle world. They then become sort of secret allies to Harry, helping him out when he's in the Muggle neighborhood.
A plot twist could be that Dumbledore faked his own death to test Harry's loyalty. But when Harry starts to turn on him, Dumbledore realizes his plan backfired. This would create a lot of tension and conflict between them, and also force Harry to re - evaluate his entire relationship with Dumbledore.
A possible plot twist could be that Odin has an ulterior motive for Harry being his grandson. Maybe he wants Harry to bring about a new order in the wizarding world that aligns with the Norse gods' plans. Another twist could be that the Death Eaters somehow find out about Harry's connection to Odin before he does and try to use it against him. They could attempt to steal Odin's power from Harry or use it to manipulate him into joining their side.
One possible plot twist could be that Harry's siren - raised nature makes him a target for dark wizards who want to use his unique abilities for their own gain.
One plot twist could be that Harry is actually a double agent. He pretends to be fully under Voldemort's control while secretly working against him. Another could be that Voldemort's training awakens a hidden power in Harry that is neither good nor bad, but something completely unexpected that changes the whole dynamic of the wizarding world.
One possible plot twist could be that Lucius Malfoy has a secret alliance with Dumbledore all along and saves Harry to fulfill some greater plan against Voldemort. Another could be that Narcissa Malfoy, out of a mother's love for all children (not just her own), decides to protect Harry. Or perhaps Draco Malfoy betrays Voldemort's orders to kill or capture Harry because he has developed an unexpected friendship with Harry during their years at Hogwarts.
Another plot twist could be that the magic in the Harry Potter universe is actually a precursor to the Force. Harry Potter discovers that his magic has the potential to evolve into the Force - like powers. This revelation makes him question his place in the wizarding world and whether he should pursue a different path, one that is more in line with what Luke Skywalker represents in the Star Wars universe.
A plot twist might be that when Harry goes to Voldemort, he discovers that Voldemort has a secret weakness. Maybe it's related to his family or some long - forgotten magic. Harry then has to decide whether to use this weakness against Voldemort or try to redeem him. This would add a whole new layer of complexity to the story. Another aspect could be that there are other forces at work, like a third - party wizard who is manipulating both Harry and Voldemort for their own gain, and Harry has to figure out who this is while dealing with Voldemort.
One possible plot twist could be that the daughter of death is initially thought to be on the side of evil but is actually a key to saving the wizarding world. Another could be that she has a secret connection to a major character from the original series that changes everything. Maybe she's related to Dumbledore in some way.
One possible plot twist could be that the female Harry Potter's kitsune nature is kept a secret from her friends at Hogwarts for a long time, and when it's finally revealed, it causes a rift in some of her relationships because they don't understand or are afraid of her new form.